Historically, steel production utilizes blast furnace iron and a scrap charge in a Basic Oxygen Furnace ("BOF") or scrap melting in an electric arc furnace to produce ingots of cast steel for reheating and rolling into manufacturing stock. Increasingly demanding applications have led to the development of more stringent physical and chemical specifications for the final steel products.
The ladle metallurgy furnace (LMF) is an additional steel refining step that has become a widely used tool to ensure consistent conformance to the rigid steelmaking requirements set by continuous casters. This additional refining step lowers the level of elements such as sulfur and phosphorous and decreases the content of non-metallic inclusions such as alumina and various sulfide and oxide species. The LMF facilitates the efficient production of steel in that specific chemical and thermal levels are rapidly achieved for meeting continuous caster delivery schedules. In the ladle refining of steel, a surface slag covering is required to provide specific chemical and physical functions. The slag composition is designed for the different grades of steel being produced with the majority requiring desulfurization. Regardless of the chemical refining requirements of the slag, it is advantageous for the ladle slag to become as fluid as possible immediately upon tapping the metal from the furnace into the ladle.
In the LMF, electric arcs from graphite electrodes impinge on the steel surface in the refining ladle to melt the top slag for efficient molten steel refining and to provide rapid heating to quickly achieve casting temperature specifications. The resulting molten slag then exists as a homogeneous, liquid refining medium when fluidity is combined with the correct refining slag chemistry. The ladle metallurgy furnace slag can act as a component of a desulfurizing addition, a slag conditioner to deoxidize the carryover or turndown slag from the previous steelmaking process, or a synthetic slag to refine the steel composition when slag deoxidization is not necessary. Additionally, a ladle metallurgy facility may be used that does not incorporate a furnace, but is strictly a station for alloying and stirring. Additional heat may be provided by the injection of oxygen and aluminum.
In order to improve delivery times of the desired quality of steel to the steel caster refining slags must rapidly achieve fluidity upon tapping the steel from the melting furnace into the refining transfer ladle to expedite chemical processing of molten steel in the ladle. Rapid and efficient chemical refining of steel requires large quantities of lime in solution within a top slag layer to provide the high basicity needed for maximum sulfur capacity to promote sulfur and phosphorus transfer between the steel and the slag. Basicity has been traditionally defined as the slag % CaO/% SiO.sub.2 ratio (the "V-ratio") and it is well known that a highly basic fluid slag is beneficial in the removal of sulfur and phosphorus from a ladle of molten steel. With respect to the V-ratio, a number less than 1 is acidic and a number more than 1 is basic. The reaction at the slag/metal interface between calcium oxide in the slag and dissolved sulfur in molten steel produces calcium sulfide that remains stable within the top slag layer as long as a reducing chemistry is maintained. By increasing the refining slag fluidity, the effective slag interfacial area available to contact the molten steel surface is likewise increased.
A ladle refining slag addition based on lime may additionally contain a variety of materials including but not limited to fluorspar, alumina (from a variety of different sources including bauxite and recycled materials such as pit solids), silica, iron oxide, titanium dioxide, sodium oxide, magnesia, calcium aluminate, limestone and dolomitic limestone, metallic deoxidizers such as aluminum, silicon, and manganese and desulfurizers such as calcium, sodium and magnesium.
The calcium oxide used is generally soft burnt lime as the reactivity of this lime is increased due to its open pore structure and is superior to hard burnt lime, which tends to develop a harder, less porous surface and is therefore less reactive in a refining slag addition. The lime must be continually fluxed to achieve maximum refining rates within the slag. As calcium sulfide forms on the surface of the lime during the desulfurizing reaction, it forms a coating which effectively seals off the remaining inner core of lime from participating in further reaction with sulfur. This not only limits the reaction rate but also limits the reaction efficiency as the inner portion of the lime particle which never enters into the desulfurizing reaction. The calcium sulfide product must be fluxed off of the remaining calcium oxide core on each lime particle for the reaction to continue at maximum rates. Since lime alone has a melting point of approximately 4700.degree. F., either a powerful heat source such as an electric arc or chemical fluxing methods must be employed to render the lime fluid.
In addition to calcium sulfide acting as a barrier against furthering the desulfurizing reaction, lime in the slag may additionally be coated with a layer of dicalcium silicate from a reaction with silica in the slag. The melting temperature of this refractory compound is around 3800.degree. F. and therefore must be prevented from forming on the surface of the individual lime particles or must be fluxed off of the lime to prevent the lime from being essentially deactivated and prevented from going into solution.
A fluid slag is best utilized for desulfurizing when it is vigorously mixed with the steel through dynamic physical particle interaction. The full body of the separate slag and metal masses may enter the slag/metal reaction interface where chemical refining is most rapid. Fluidity and slag/metal mixing therefore effectively increases the interfacial area of the slag to accelerate the refining reactions.
Steelmaking temperatures are traditionally 2700.degree. F.-3000.degree. F. Accordingly, solvents for taking lime into a liquid state must be employed to create a final slag product that has melting point beneath this threshold level. To facilitate maximum refining rates, the dissolution of lime is required immediately upon tapping the melted steel into the ladle. In this regard, agents which provide this physical fluxing effect while remaining compatible with the chemical requirements of the slag are required. The development of slag-making additions with the attendant means to flux lime has been widely practiced.
Fluorspar has traditionally been the most commonly used fluxing agent for lime. However, its corrosive effect on ladle refractories as well as its reaction with carbon and silicon to produce environmentally and physically harmful carbon tetrafluoride and silicon tetrafluoride has led to its elimination from many processes and a search for a replacement flux in the ladle refining of steel. The production of these harmful compounds also causes the fluorine in the slag to evaporate in the gaseous form and thereby depletes the slag of this fluidizing substance. The top slag layer will therefore become very stiff and unworkable if left too long on top of the steel.
Alumina on the other hand is very stable and when fluid, can chemically combine with lime to form dicalcium aluminate which has a melting point of around 2550.degree. F. The chemical combination of CaO and Al.sub.2 O.sub.3 in equal weights creates the lowest melting point mixture possible so that it stays fluid at steelmaking temperatures. Any deviation from this 50/50 balance will cause the melting point of the material to increase unless other oxide impurities are present in which case the melting point is again decreased.
Premelted calcium aluminates have been employed as synthetic steelmaking slag desulfurizers since around 1937 in France, when Rene Perrin utilized a separate furnace to provide a molten addition of 50% calcium oxide and 50% aluminum oxide to a ladle of molten deoxidized steel which achieved an 80% drop in sulfur from 0.025% to 0.005% during furnace tapping. This slag addition process contained the necessary desulfurization ingredients of high temperature, high basicity and low oxygen potential with vigorous stirring into the deoxidized steel. Although greatly successful in desulfurizing, the cost of the process is prohibitive.
The addition of a premelted but solid calcium aluminate is very effective in slag fluxing and is usable as a desulfurizing agent. However, it too is very expensive. Lower cost substitutes include the use of a byproduct of ferrovanadium manufacture that consists of a large percentage of premelted lime and alumina along with vanadium pentoxide. Although this product does not have a 50/50 weight ratio of lime to alumina, the other tramp or other lower melting point oxides help to decrease the melting point considerably. This is undesirable since vanadium can be unacceptable in cerain grades of steel.
Different techniques have been used to combine lime and alumina on a molten steel surface and allow the electric arc of an electric melting or refining furnace to melt and fuse the two compounds together. An approach using the generation of chemical heat within a self-fluxing desulfurizing addition is detailed in U.S. Pat. No. 4,342,590 where finely divided iron oxide is mixed with aluminum and lime. The heat generated by the reaction between aluminum and the oxygen in the iron oxide helps to melt the lime and the resulting aluminum oxide into a calcium aluminate. This Thermit reaction is very effective for heat generation but is expensive since aluminum is consumed by the iron oxide additions to provide the thermal reaction for lime melting and fluxing.
Silica additions also cause a fluxing action on lime with the resulting substance being a lime-silica wollastonite compound. U.S. Pat. No. 4,695,318 refers to a premelted calcium silicate and fluorspar being used to flux lime in a ladle refining process. This is not acceptable in many refining slag practices as silica is not only detrimental to desulfurizing performance but is also acidic in nature and is very corrosive to the basic refractories used in refining ladles. High silica levels in a refining slag are also known to be deleterious to cleanliness levels in aluminum treated steels.
Another chemical flux for lime-containing slags is liquid iron oxide. Iron oxide is naturally present in steel melting furnace slags and typically exists in quantities of between 10% and 40% in BOF slags. As oxygen is blown into the furnace charge of molten high carbon iron and steel scrap to remove the carbon by formation of carbon monoxide, large quantities of iron are also oxidized and float into the melting slag. When the steel melt is tapped into the transfer ladle, a significant amount of melting slag (also known as carryover or turndown slag) is unavoidably carried over into the ladle with the steel. Although this flux will rapidly cause the lime to go into solution in the slag, its highly oxidizing nature is detrimental to alloy recovery in the steel and to the desulfurizing capacity of the slag. BOF furnace melting slag contains large quantities of highly oxidizing iron oxide that must be physically removed or chemically reduced for desulfurization to continue.
Although large quantities of iron oxide from BOF furnace slags are to be avoided, a smaller, controlled addition of iron oxide in solid form has been used as a flux for high lime quantities in ladle slag additions. Similarly, as taught by U.S. Pat. No. 3,964,899, iron, manganese, titanium and aluminum oxides as well as fluorspar obtained from pit solids waste can be used to flux lime in a furnace refining operation. In this patent, pit solids are described as a waste material from aluminum production containing large percentages of alumina with lesser amounts of lime, silica and magnesia.
The addition of lime with metallic aluminum has been also used to obtain a desulfurizing slag by providing aluminum as a means to absorb the oxygen released from the calcium oxide when it becomes displaced by the sulfur. This effectively removes the released oxygen from the reaction zone and prevents it from further reacting with the sulfide product and causing sulfur reversion from the slag to the steel.
U.S. Pat. No. 4,142,887 describes a ladle desulfurizer where a mixture of particulate metallic aluminum, fluorspar and lime is added to deoxidize and desulfurize the steel while forming a fluidized slag. This slag can then act to further deoxidize and desulfurize the steel upon subsequent mixing but can additionally provide atmospheric coverage and protection to prevent reversion of the removed sulfur back into the steel melt. U.S. Pat. No. 4,060,406 refers to a slag conditioner for electric arc steelmaking whereby the addition comprises aluminum, alumina and fluorspar and an alkali metal carbonate. This addition specifies that the raw materials employed should ideally be low in sulfur, phosphorus and iron oxide but that a variety of materials are available commercially including waste materials such as pit solids and ball mill dust.
A synthetic slag addition is generally required if iron oxide levels in the slag are lower, such as when higher carbon BOF steels are produced or when electric furnace melting furnaces are tapped with a minimum of carryover slag. This provides coverage to the ladle surface for atmospheric protection of metallic alloy additions such as aluminum and silicon which have been added to the steel to produce specified levels of these elements. Readily available oxygen sources such as the atmosphere or a top slag high in weak oxides will cause the alloying elements to be oxidized into the slag requiring an expensive re-alloying procedure.
This synthetic slag addition therefore provides atmospheric protection and dilutes the carryover slag so that the weak acidic oxides in the melting slag are not available to oxidize the alloys dissolved in the steel bath. Thermal insulation of the molten bath is additionally provided by the slag as rapid heat loss via radiation from the steel is blocked by the slag.
Physical refractory protection from the electric arc in the ladle refining furnace is facilitated by the top slag. If a surface slag layer is not used in the arc refining of steel, the electric arc will experience flare and rebound off the steel and refractory lining in the ladle in a manner severe enough to cause a break in the ladle refractory with the attendant loss of steel from the ladle. The top slag is therefore required for physical protection of the steel chemical levels and ladle refractories.
Additionally, slag fluidity is essential in modern steelmaking to promote absorption of nonmetallic inclusions from the steel bath since inclusions will cause clogging of casters and result in production losses and increased costs. Nonmetallic inclusions also cause surface defects in the final rolled steel, making a lower quality steel.
Synthetic slag patents have referred to the use of recycled waste materials in the fluidization of lime-rich refining ladle slags. U.S. Pat. No. 3,320,052 teaches the use of a particulate dust collected from aluminum production as a flux for steelmaking slag additions. This material contains lime, fluorine, alumina and cryolite (sodium aluminum flouride) and was processed, mixed and sometimes briquetted for production of a larger size to facilitate handling. U.S. Pat. No. 4,039,320 describes a briquetted mixture of aluminum and lime for addition to a melting furnace during the reducing period in the refining process in the making of stainless steel in order to chemically reduce the acidic oxides in the slag and increase the basicity of the slag. Metallic calcium bonded to a calcium aluminate flux is described in U.S. Pat. No. 4,435,210 as an addition for the deoxidation, desulfurization and dephosphorization of molten steel by ladle refining. Waste slags from melting and oxidizing furnaces have been described in differing applications for molten steel refining. U.S. Pat. No. 4,364,771 mentions the use of a granulated slag with a V-ratio higher than 1 in a mixture with magnesium shot for injection into pig iron to desulfurize and nodularize the metal. The granulated slag acts as a flow promoter in the injection process and facilitates desulfurizing within the metal. Recycled slag use is outlined in U.S. Pat. No. 2,361,416 whereby recycled cupola melting slag is used as a flux for limestone in the subsequent furnace melt. Specified is the requirement for a measure of fresh limestone with each slag charge to dilute the impurities in the recycled slag. The addition is detailed as combining recycled slag containing more than 35% silica with a lesser amount of fresh limestone. U.S. Pat. No. 3,897,244 describes using slag a means of fluxing dicalcium silicate from lime particles used in a basic oxygen furnace operation. The slag could be obtained from electric furnaces, open hearth furnaces or basic open hearth converters. In order to act as a flux, the preferred amounts of iron oxide and silica in the slag were 15-30% and 10-16% respectively. This fluxing addition helped the improve the refining reaction during the carbon blow stage of refining.
Ladle refining has been investigated as an area where slag recycling could be used to benefit the fluxing into solution of lime. U.S. Pat. No. 4,842,642 details the use of blast furnace slag as a flux in a ladle refining operation which therefore allows for the elimination of fluorspar. The blast furnace slag employed is described as containing 5-15% alumina and 30-45% silica along with 0-2% phosphorus and 1-2% sulfur. This patent suggests that the mechanism of fluxing is chemical dissolution of the lime into the already-molten recycled slag layer upon addition of the mix to the ladle of molten steel as the slag melts around 2400.degree. F., which is considerably below steelmaking temperatures.
In Japanese patent application No. P-96322, molten steel is refined using a ladle having recycled molten slag remaining from a preceding heat of refining. A small quantity of solid alloy materials and raw slag making material are added to the ladle containing the molten steel and recycled molten slag to form a new slag layer. In this process a lid is placed on the ladle to seal the interior of the ladle and then argon gas is blown into the molten steel so as to stir the molten steel and reduce the atmosphere inside the ladle. Japanese application No. P-96322 uses sensible heat of the molten recycled slag to melt the raw materials into the molten steel and slag. This Japanese process is disadvantageous in that a steel manufacturer can not accurately control the chemistry and weight of the recycled addition. Accordingly, when incorporating raw materials into the molten slag, the raw material effectiveness is diminished through reactions between the raw materials and the molten recycled slag as opposed to reactions between the added raw materials, the steel and the slag in the ladle. Additionally, the Japanese process does not allow for the transfer of the LMF slag to other steel making locations since the LMF slag must remain molten to provide the sensible heat required.
It is therefore an object of the present invention to overcome the problems in the prior art and to provide a material suitable for use as low melting temperature slag-making material that is inexpensive. It is a further object of the present invention to formulate a slag-making composition that achieves a low melting point that is equivalent to calcium aluminate with respect it its fluxing potential, while providing a greater capacity to absorb non-metallic inclusions into the top slag solutions when these products of steel deoxidation and desulfurization are floated into the interface between the steel and slag. It is an object to provide an addition that avoids excessive silica, sulfur, and phosphorus levels, and to permit the addition of the ladle additive both during and after tapping the steel from the furnace into the ladle. Furthermore, the present invention avoids the undesirable levels of titanium and vanadium that may be present in calcium aluminate and vanadium slags and further limits the amount of weak acidic oxides and fluorspar as fluxes for the lime since these are detrimental as discussed above.
Moreover, because the LMF slag of the present of the invention is a solid, it permits the precise control in terms of chemistry and weight of each component in the addition. The present invention can also, therefore, be added during or after the molten steel is tapped into ladle. Because the present invention can be added at various stages of tapping, it can be taylored to act specifically upon the steel, such as in desulfurizing, or on the carryover slag in the slag conditioning application. By using the present invention to maximize slag fluidity upon tap, decreased LMF processing times affords a decrease in power consumption and associated wear on the ladle refining system. These processing and handling advantages facilitate timely delivery of the steel to the caster.
Additionally, the present invention can be used by steel manufacturers that do not have an in-house source of the desired LMF slag.